The present invention relates to fuses for protecting electric wires, equipments and so on from an overcurrent.
In electric circuits of automobiles or the like, conventionally, fuses were used to protect electric wires, equipments and so on from an overcurrent. JP-A-7-130277 discloses such a fuse in which a thin extension portion is formed in a fusible portion to improve the effect of collecting heat so as to obtain superior fusing-off property. In this fuse 1, as shown in FIG. 1, base portions 5 are provided on a pair of terminals 3 and 3 so as to project therefrom, and caulking protrusions 7 are provided at these base portions 5. The base portions 5 are connected to each other through a slender extension portion 9. After the opposite ends of a low-melting chip are fixed by the caulking protrusions 7, the low-melting chip is heated so as to be melted once, so that cohesive portions 13 and 13 which are a pair of increased-sectional-area portions are formed around the caulking protrusions 7 by the surface tension, while a thin extension portion 15 is formed at the center of the slender extension portion 9.
According to the thus configured fuse 1, the sectional area of the slender extension portion 9 is smaller than the sectional area of each of the cohesive portions 13 and 13 so that a sufficient constriction rate can be obtained. By the heat collecting effect of the constriction, it is easy to specify the position of a fusing portion and reduce a heating area. Accordingly, it is possible to specify a hot spot position and reduce a heating area, so that not only it is possible to restrain a fuse element from heating as a whole so as to reduce unnecessary heat transfer to the terminals 3 and 3 of a housing not shown and the surroundings thereof, but also it is possible to obtain superior fusing-off property in a low current range by effective use of heating.
However, generally, a fuse has a constant correlation between current conduction and fusing-off time. That is, while a fusible portion of a fuse element is fused off immediately in the case of short (dead short-circuit), for example, caused by a current of 200% or more of the rated current of the fuse, the fusible portion repeats heat generation and radiation in the case of a short-circuit caused by a current of 200% or less of the rated current of the fuse or in an intermittent short-circuit (layer short-circuit), so that there is a tendency to increase the fusing-off time. Under such conditions, electric wires constituting a circuit do not radiate heat as the fusible portion due to an insulating coating covering it even if the intermittent short current flows in the wires. Therefore the temperature continues to increase because of heat accumulation, and there is a fear that smoking or the like may be caused in the worst case.
For solving such a problem, there is a method in which a base material of a fuse element is diffused into low-melting metal to make the fusing-off temperature lower than the melting point of the base material to thereby shorten only the fusing-off time in a layer short-circuit range without changing the fusing-off property in a dead short-circuit range so as to improve the fusing-off property in the layer short-circuit range.
Since the thin extension portion 15 is formed at the center of the fusible portion in the above-mentioned conventional fuse 1 in order to increase the constriction rate, it is possible to obtain a heat collecting effect based on the constriction. It is, however, impossible to obtain a sufficient fusing amount of the low-melting metal, and it is therefore difficult to obtain an effect to shorten the fusing-off time on the basis of the diffusion.
In addition, in the above-mentioned conventional fuse 1, a low-melting chip is caulked by the caulking protrusions 7 after the terminals 3 and 3, the base portions 5 and the slender extension portion 9 are formed by pressing, and the low-melting chip is heated and melted to be welded to the slender extension portion 9. There is, therefore, a problem that the steps of coating with flux before welding, caulking, heating and melting the low-melting chip, cleaning the flux after welding, and so on are complicated so that the manufacturing cost is high.